The present invention relates to a voltage regulator with a low drop-out voltage, which can be linked with a power supply, which may be temporarily interrupted.
The present invention is more particularly intended for use in automobile electronics. There is an increasing tendency to use microprocessors in cars for controlling or supplying information. These microprocessors require a stable supply voltage, e.g. 5 V to within 0.25 V. However, car batteries which normally supply a voltage of approximately 12 to 14 V may, under certain circumstances, e g. during cold starts, be greatly discharged and supply low voltages of e.g. only 5.5 V. Moreover, the electrical environment of a car engine is greatly subject to interference and in circuits there can be high overvoltages up to 120 V in positive and negative peak values. When this is a negative peak value, this means that the power supply is interrupted. Fortunately, these interruption periods are brief and only last a few milliseconds.
FIG. 1 shows a simple example of a regulator, with a low drop-out voltage. This regulator, which receives a supply voltage V.sub.A between its input terminals 1 and 2, e.g. from a battery, supplies a use voltage V.sub.U between its output terminals 3 and 2, e.g. for a microprocessor. It essentially comprises a pnp transistor 4, whose emitter is connected to the input terminal 1 and whose collector is connected to the output terminal 3. The base of the pnp transistor 4 is supplied by a regulation control circuit 5. The latter comprises e.g. a differential amplifier 6 receiving on the one hand a reference voltage at its first input terminal 7, and on the other hand at its second input terminal 8 a signal from a divider bridge 9, 10, which converts the voltage present between output terminals 3 and 2. For example, when the use voltage must be 5 V, the supply voltage must drop to a value of 5 V+V.sub.CEsat, V.sub.CEsat being the saturation collector-emitter voltage drop of the pnp transistor and which can be approximately 0.4 to 0.5 V, when the base voltage is suitable. Thus, the difference between the use voltage and the minimum supply voltage, i.e. the drop-out voltage of the regulator is well below 1 V, even when account is taken of the losses in the electrical wires.
Consideration will now be given to how it is possible to retain the advantages of a low voltage drop of the pnp transistor--equipped regulator, whilst making it possible to maintain the use voltage during the brief drops of the supply voltage below the threshold value.
Whilst using the same references as employed in FIG. 1 for the same components, FIGS. 2 and 3 illustrate attempts made in the prior art to achieve this aim.
In the case of FIG. 2, a storage capacitor 11 is placed between the supply terminals 1, 2 and a diode 12 prevents said capacitor from discharging towards the power supply. Thus, as soon as the voltage between terminals 1, 2 temporarily drops, the capacitor comes into action and supplies the circuit. This operates well with reasonable capacitance values of capacitor 11, when the power supply is normally at a high level and temporarily drops, e.g. for a few milliseconds to low values. However, this configuration does not enable the supply voltage to remain at values close to the use voltage for relatively long periods. Thus, the voltage drop in normal operation between terminals 1 and 3 consists, as hereinbefore, of the saturation collector - emitter voltage drop of the pnp transistor 4 (V.sub.CEsat), but it is necessary to add thereto the voltage drop in diode 12 of approximately 0.6 to 0.8 V, which gives a total exceeding 1 V.
In order to obviate this disadvantage, the configuration of FIG. 3 is proposed, in which a storage capacitor 13 is placed between the use terminals. In this case, normal operation is satisfactory, because then the potential difference between the supply terminals 1 and 3 is only the collector - emitter voltage drop of transistor 4. However, the storage capacitor 13 is normally supplied at the use voltage, so that, necessarily, as soon as the supply voltage is interrupted, the voltage between the use terminals drops. For example, it is desired that this voltage cannot drop by more than 0.25 V. A simple calculation shows that in practice, for this purpose and if the voltage interruptions last a few milliseconds, values are obtained for the capacitor, which are prohibitive for a normal consumption of the integrated circuit, e.g. approximately 150 mA.
Thus, the prior art teaches us that regulators using storage capacitors have one or other of the two aforementioned disadvantages. Thus, either the drop-out voltage (minimum difference between the supply voltage and the use voltage) is too high, as in the case of FIG. 2, or it is necessary to provide capacitors having an excessively high value, which are expensive, have large overall dimensions and have poor thermal drift characteristics, as in FIG. 3.